1. Field of the Invention
The present invention relates to an AC generator for use in motor vehicles, and more particularly to improvement of a rectifier and its associated parts.
2. Description of the Related Art
FIG. 15 is a side-elevational and cross-sectional view showing a construction of a prior motor vehicle AC generator disclosed in, for example, Japanese Unexamined Patent Publication No. 8-182279, FIGS. 16 and 17 are respectively a front elevational view and a rear elevational view each showing a rectifier of the prior motor vehicle AC generator, and FIG. 18 is a cross-sectional view taken along a line XVIII--XVIII of FIG. 16.
The prior motor vehicle AC generator shown in FIG. 15 is composed of a casing 3 comprising a front bracket 1 and a rear bracket 2 each made of aluminium, a shaft 6 rotatably supported by the casing 3 and fixedly holding a pulley 4 at its front bracket 1 side end portion, a lundell type rotor 7 fixedly secured to the shaft 6, fans 5 respectively fixed to both the side surfaces of this rotor 7, a stator 8 fixedly secured to an inner wall surface of the casing 3 so as to surround the rotor 7, a slip ring 9 fixedly secured to a rear-bracket 2 side end portion of the shaft 6 for supplying a current to the stator 8, a pair of brushes 10 housed within a brush holder 11 fixed to the rear bracket 2 and made to slide on the slip ring 9, a rectifier 12 electrically connected to the stator 8 for rectifying an alternating current generated in the stator 8 into a direct current, and a regulator 18 fitted in the brush holder 11 for regulating the magnitude of an alternating current generated in the stator 8.
The rotor 7 is made up of a rotor coil 13 for generating a magnetic flux in response to the flow of a current and a field (pole) core 14 in which a magnetic pole is formed by the magnetic flux generated by the rotor coil 13. Further, the stator 8 is composed of a stator core 15 and a stator coil 16 configured by winding a conductor line around the stator core 15 for producing an alternating current due to the variation of a magnetic flux from the rotor coil 13 in accordance with the rotation of the rotor 7.
In the motor vehicle AC generator thus constructed, a current is supplied from a battery (not shown) through the brush 10 and the slip ring 9 to the rotor coil 13 so that a magnetic flux occurs in the rotor coil 13. A rotational torque of an engine (not shown) is transferred to pulley 4 to rotationally drive the shaft 6, thereby rotating the rotor 7.
Whereupon, a rotating magnetic field is given to the stator coil 16 so that an electromotive force develops in the stator coil 16. This ac electromotive force enters the rectifier 12 to be rectified into a direct current, and after its magnitude is regulated by the regulator 18, is used for charging the battery.
Referring now to FIGS. 16 to 18, a description will be made hereinbelow of a concrete construction of the rectifier 12.
This rectifier 12 comprises a positive-polarity (pole) side cooling plate 21 on which diodes 22 are mounted as positive-polarity side one-way conducting devices, a negative-polarity side cooling plate 23 on which diodes 24 are mounted as negative-polarity side one-way conducting devices, and a circuit board 25.
Furthermore, the positive-polarity side cooling plate 21 is molded out of an aluminium alloy by the die-cast method to have a substantially arc and strip configuration, and its one side is formed as a flat fitting surface 21a while a radiating fin 21b radially protruding is placed on the other side. Overlapping portions 21c are located at both end portions and central portion of the cooling plate 21 in the arcuation directions to extend radially and outwardly, and fixing holes 21d are bored in the overlapping portions 21c. In addition, four diode fitting holes 21e recessed to form a rectangular configuration are radially provided in the fitting surface 21a of the cooling plate 21, and the diodes 22 are soldered therein. On each of the diodes 22, a lead 22a is bent to form an L-shaped configuration, and a base electrode surface thereof is soldered in the diode fitting hole 21e to excellently maintain in an electrically and thermally contacting condition in a state where the lead 22a turns radially and outwardly.
Still further, the negative-polarity side cooling plate 23 is molded in an aluminium alloy according to the die-cast method to have a substantially arc strip configuration with a radius of curvature larger than that of the positive-polarity side cooling plate 21, and its one side is formed as a flat fitting surface 23a while the other side is formed as a flat direct-fitting surface 23b. In this cooling plate 23, fixing holes 23c are bored to overlap with the fixing holes 21d of the cooling plate 21. Further, four diode fitting holes 23d recessed to make a rectangular configuration are radially formed in the fitting surface 23a of the cooling plate 23, and the diodes 24 are soldered therein. On each of the diodes 24, a lead 24a is bent to form an L-shaped configuration, and a base electrode surface thereof is soldered in the diode fitting hole 23d to excellently maintain in an electrically and thermally contacting condition in a state where the lead 24a is directed in a circumferential direction.
Moreover, in the circuit board 25, four connecting terminals 25a made from a steel plate are integrally formed through an insulating material to form a substantially arc and strip configuration, and serve as a relay to electrically establish connections to the stator coil 16, the regulator 18 and the diodes 22, 24. Incidentally, in the illustration, numeral 27 designates an output terminal.
For assembling this rectifier 12 thus constructed, the diodes 22, 24 are first soldered in the diode fitting holes 21e, 23d of the cooling plates 21, 23. Subsequently, the overlapping portions 21c of the cooling plate 21 are overlapped with the cooling plate 23 in a state where an insulating member 28 is interposed therebetween, and the circuit board 25 is placed thereon. Following this, the connecting terminals 25a are connected to the leads 22a, 24a by the spot welding, thereby providing the rectifier 12.
Thereafter, the rectifier 12 thus assembled is inserted into the rear bracket 2, and the direct-fitting surface 23b of the cooling plate 23 is located through a silicone sheet 29a on a direct-fitting surface 2a of the rear bracket 2. In this state, fitting screws 26 are inserted into the fixing holes 21d, 23c and holes 2b made in the rear bracket 2 from the circuit board 25 side and their protruding portions are tightened with fitting nuts 30, respectively, so that the rectifier 12 is attached to the rear bracket 2. At this time, the fixing hole 23c of the cooling plate 23 has a female screw, and when the fitting screw 26 is engaged with the fixing hole 23c, as indicated by an arrow in FIG. 18, a current path is constituted by the fitting screw 26 between the rear bracket 2 and the cooling plate 23.
Besides, in the prior motor vehicle AC generator, although a female screw is made in the fixing hole 23c of the cooling plate 23 and the electric conductivity is secured by the screw coupling between the fitting screw 26 and the fixing hole 23c, it is also acceptable that the electric conductivity is secured by the knurling coupling between the fitting screw and the fixing hole.
Since, as described above, in the prior motor vehicle AC generator, the rectifier 12 is fixedly supported on the rear bracket 2 in a manner that the silicone sheet 29a is put between the entire surface of the direct-fitting surface 23b of the cooling plate 23 and the direct-fitting surface 2a of the rear bracket 2, the whole faying surfaces of the cooling plate 23 and the rear bracket 2 are brought closely into contact with each other in a state where the silicone sheet 29a excellent in thermal conductivity is interposed therebetween, thereby securing the thermal conductivity between the cooling plate 23 and the rear bracket 2.
However, since the silicone sheet 29a has a high insulation performance, the faying surfaces of the cooling plate 23 and the rear bracket do not provide adequate electrical conductivity 2. More specifically, for securing the electric conductivity between the cooling plate 23 and the rear bracket 2, there is a need to provide a female screw in the fixing hole 23c of the cooling plate 23, and to tighten the fitting screw 26 into the fixing hole 23c and further to bring the fitting screw 26 and the rear bracket 2 into contact with each other through the use of a fitting nut. In addition, since an electric conducting medium such as the fitting screw 26 is needed for securing the electric conductivity, a problem occurs in that the electric resistance increases between the cooling plate 23 and the rear bracket 2.
Moreover, although there is a different fitting way to remove the silicone sheet 29a and to directly fay the direct-fitting surface 23b of the cooling plate 23 and the direct-fitting surface 2a of the rear bracket 2 with each other, because the rear bracket is manufactured out of aluminium according to the die-cast method, the surface roughness is as high as 20 to 30 .mu.m, which makes it difficult to completely and closely bring the cooling plate 23 and the rear bracket 2 into contact with each other without using an intervening member or material, with the result that the thermal conductivity deteriorates. Further, because of no complete close contact between the cooling plate 23 and the rear bracket 2, salt water or dust from the external environment tends to enter there increasing the electric resistance due to the occurrence of rust.